A contemporary challenge for many social sciences is the integration of biological and socioeconomic approaches in a more unified framework that (a) acknowledges the role of biological endowments, (b) recognizes biologically mediated variation in psychological characteristics and personal behaviors, and (c) maintains at the same time a possibly strong role for conscious and rational decision-making in a life-cycle perspective. Behaviors related to fertility are primary candidates for this integration. In the proposed project, we will therefore use large-scale, population-based and longitudinal Danish twin data on fertility, fertility related behaviors and well-being to empirically investigate and theoretically conceptualize bio-social determinants of fertility and related behaviors. The data for these analyses will be partially collected in the course of the proposed project, but much of the project involves existing data already available in the Danish Twin Registry. Our analyses of these data will (1) establish the genetic component of the variance in fertility and related behaviors, including investigations of gene-environment interactions and long-term cohort comparisons; (2) provide a decomposition of this genetic contribution into different pathways, such as through education/human capital accumulation, union formation or labor market participation; (3) estimate structural models for fertility, investments in child-quality, union-formation, labor force participation that control for biological and other endowments using within-MZ estimates; (4) estimate the contributions of children (and marriage) to subjective well-being and happiness, after controlling for biological and other endowments, in order to better understand the motivation of why individuals desire to have children; (5) analyze biological influences on social interaction processes among twins that affect fertility, partnership formation and happiness derived from children; and (6) investigate the impact of the "shock" of having twins on subsequent family outcomes in order to study the implications of quantum-variations on investments in child-quality, parent's human-capital accumulation and labor-market behavior within evolutionary and economic allocation models.